Field of the Invention
The invention disclosed in the present specification relates to a laser beam irradiation system utilized in annealing a semiconductor thin film and to an application thereof.
Hitherto, there has been known a technology for annealing a semiconductor thin film in various ways by irradiating a laser beam. For example, there has been known a technology for obtaining a crystal silicon film by forming an amorphous silicon film on a glass substrate by means of plasma CVD and the like and then by irradiating thereon a pulse oscillating excimer laser beam oscillating in an ultraviolet range in fabricating a thin film transistor on the glass substrate.
The excimer laser is used because it provides an irradiation energy density and wavelength which are suitable for annealing the silicon film.
However, because the normal laser light is a spot-like beam of around several mm square, there has been a problem in terms of productivity when a glass substrate having a size of several tens cm square is used.
Then, in order to solve this problem, there has been a technology for dealing with such a large area by processing the laser beam into a linear beam of several tens cm in length by an optical system and by scanning such linear laser beam (called a linear laser).
However, there has been a problem in fabricating a thin film transistor by the method using the linear laser. A thin film transistor is fabricated not as a simplex body but as an integrated body in fabricating it. For example, when thin film transistors are to be used in an active matrix type liquid crystal display to which peripheral driving circuits are integrated, circuits which compose the peripheral driving circuits such as shift registers, buffer circuits and analog switching circuits are disposed by repeating the same circuit patterns.
When the linear laser beam is utilized to obtain such a structure, fine stripes are seen in an image on the liquid crystal display thus obtained. The stripes are categorized into two types of stripes whose longitudinal directions cross at right angles each other, i.e. into vertical and horizontal stripes.
It is noted that the vertical and horizontal stripes are seen even when the laser scanning direction is turned by 90 degrees.
The stripes are perceived as crystal non-uniformity also when a silicon film is observed after annealing.
According to the finding of the inventor, et. al., the vertical and horizontal stripes described above are related to dispersion of irradiation energy density of the linear laser beam in the longitudinal and scanning directions.
The dispersion of the irradiation energy density of the linear laser beam in the longitudinal direction is caused by dispersion of discharge starting points within the oscillator. That is, it is caused as a result of deviation of a distribution of density of the laser beam emitted from the oscillator magnified by the optical system. Because the linear laser beam is what is shaped into the beam having several mm in width and several tens cm in length from the spot beam of several cm square by the optical system, the dispersion of the discharge points within the oscillator is magnified considerably.
Then, the dispersion of the irradiation energy density of the linear laser beam in the longitudinal direction may be considered to be caused by the spatial dispersion of the oscillating positions within the laser oscillator.
In concrete, in the structure of a known prior art excimer laser oscillator shown in FIG. 1B, the oscillating positions (or more accurately, the oscillation starting positions) are dispersed in the longitudinal direction of an oscillator case per oscillation because gas is introduced from one end of the oscillator as indicated by an arrow 315.
In the structure shown in FIG. 1B, the reference numeral 311 denotes the oscillator case, discharging electrodes 312, a fan 313 driven by a motor 314, and a laser beam 316 emitted to the outside.
Even in such a structure, the spatial dispersion of the oscillating positions within the laser oscillator described above would not exert a large influence when a spot light (a circular beam or a rectangular beam whose difference of proportion of vertical length and horizontal length is small) is utilized.
That is, the influence would not be actualized as the dispersion of the irradiation energy density within the laser beam irradiated to a surface to be irradiated because the laser beam is not largely magnified and the dispersion of the oscillating positions hardly appears as the difference of irradiation energy density within the beam.
The dispersion of the irradiation energy density of the linear laser beam in the scanning direction is caused by the temporal stability of the oscillator, i.e. the dispersion of the irradiation energy density per oscillation. It may be considered to be caused by the dispersion of temporal oscillation intensity of the laser oscillator.
The dispersion of the temporal oscillation intensity of the laser oscillator may be improved considerably by optimizing a gas circulating method, an oscillation frequency and a scanning speed.
However, it has been clarified that the spatial dispersion of the laser oscillation is caused by the structure of the laser oscillator and the oscillating method thereof and that it cannot be suppressed just by optimizing the conditions.